1. Field of the Invention
This invention relates to a fuel-cell system having a fuel battery comprising more than one fuel cell that changes chemical energy of a fuel and oxidant to electric energy.
2. Description of the Related Art
Retention and/or accumulation of nonreactant gas and/or generated water in a fuel battery lowers the ability to generate power. The practice is therefore to purge the fuel battery of nonreactant gas and/or generated water at regular time intervals, while also monitoring the generated voltage of the fuel battery and carrying out a supplementary purge whenever the generated voltage falls below a predetermined level, as disclosed, for example, in Japanese Laid-open Patent Application No. 2000-243417.
The amount of nonreactant gas and generated water present in a fuel battery varies with the operating state of (load on) the fuel battery. When purges are programmed to be conducted at predetermined time intervals (time periods between consecutive purges) as in the prior art, however, purge is not always performed at the optimum time point. This may result in accumulation of retained nonreactant gas and/or generated water because purge is not carried out when necessary or in repeated unnecessary purges.
On the other hand, the method of conducting purges when the output voltage falls below a predetermined level is also liable to result in repeated unnecessary purges when the fuel battery is connected to a high-load external piece of equipment and the output voltage is stable at a low level.
Aside from the above, when the operation (power generation) of a fuel battery is terminated or stopped, nitrogen and other nonreactant gases may be retained in the fuel battery owing to air flowing in from the air electrode through the electrolytic membrane. In addition, generated water produced in the course of operation sometimes collects to cause water clogging of the passages and diffusion membranes. When such a situation arises, fuel (hydrogen gas) supplied to start power generation of the fuel battery is blocked by the retained gas and/or water and does not reach the fuel cell electrodes, occasionally making it impossible to obtain a desired electromotive force.
It is therefore a conventional practice to discharge (purge) residue from a fuel battery when the operation is to be started, as disclosed, for example, in Japanese Laid-open Patent Application No. 2001-216988.
Most purge systems for discharging residue are composed of a passage (piping/tubing) communicating with the exterior of the fuel battery and a solenoid valve for opening and closing the passage. Moreover, the fuel supply system for supplying fuel (hydrogen gas) to the fuel battery is generally composed of a passage that connects the fuel battery to a gas cylinder charged with fuel gas under high pressure and a solenoid valve for opening and closing the passage. Since solenoid valves are used in the fuel supply and purge systems, an external power source such as a battery is required when the power generation is started. When the fuel battery is used as a portable power generating unit, therefore, portability is degraded by the larger volume and weight of the unit.